put one off one's stride
Also, put one off one's stroke. Interfere with one's progress, distract or disturb one, as in The interruption put her off her stride for a moment, and she took several seconds to resume her train of thought, or The noise of the airplanes overhead put her off her stroke, and she missed the next ball. The first term, first recorded in 1946, alludes to the regular pace of a walker or runner; the variant, first recorded in 1914, alludes to the regular strokes of a rower. Also see throw off the track.
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